Various fluorescent molecular probes used for analysis of a structure of a protein or a structure or a function of biomolecules such as a protein have been known. For example, the fluorescent molecular probe can be exemplified by fluorescamine specifically reacting with a primary amine and exhibiting fluorescence. Then, various types of information on a molecular structure such as a motion or an orientation of a target protein can be collected by labeling a target protein with the fluorescent molecular probe and observing the target protein with a fluorescence microscope. A nanodiamond particle has recently attracted attention as a fluorescent substance used for such a fluorescent molecular probe (for example, Japanese Patent Laying-Open No. 2011-180570 (PTD 1)).